Due to their light weight, strength, corrosion resistance, freedom of design and recyclability, magnesium alloys as typified by AZ-31 and AZ-91 are recently used in a wide variety of applications including information electronic equipment such as mobile phones, digital video cameras, digital cameras, liquid crystal projectors, plasma displays, personal computers, MD players, DVD recorders, and vehicle components, typically automotive components such as electric and electronic parts, oil pans, intake manifolds, lock housing parts, steering upper brackets, and steering wheels. It would be desirable to have an organopolysiloxane composition which is self-adhesive to magnesium alloy members.
Magnesium alloy members, however, are extremely difficult to bond. Prior to bonding, they must be modified by chemical treatment. Few studies have been made on sealing materials and adhesives which are self-adhesive to magnesium alloy members without a need for such treatment. For organopolysiloxane compositions which are self-adhesive to magnesium alloys, only a few techniques have been proposed. U.S. Pat. No. 6,645,339 (corresponding to JP-A 2003-535152) discloses a composition comprising a curable silicone, an amino-containing silane adhesion promoter, and a filler; and JP-A 2002-309219 discloses a composition comprising a silicone oil and an inorganic compound containing a metal element having a higher standard electrode potential than magnesium as a filler. As to the former, the amino-containing silane adhesion promoters such as γ-aminopropyltrialkoxysilane and trialkoxypropylethylenediamine are less effective. The latter composition, which essentially contains a hydroxyl end-capped organopolysiloxane or methoxy end-capped organopolysiloxane, lacks chemical resistance, especially durable physical properties against automobile long-life coolant aqueous solution. As used herein, the term “durable physical properties” refer to the maintenance of rubber physical properties after a chemical resistance test.
The latest technology is found in JP-A 2005-298558. For improving the adhesion to magnesium alloy of an organopolysiloxane composition, it proposes to use an acidic silane coupling agent which exhibits pH 7 or lower in a 5% aqueous solution thereof. The use of acidic silane coupling agent, however, can cause corrosion to the magnesium alloy due to the acidity thereof so that the magnesium alloy itself loses strength. After a chemical resistance test (especially in engine oil, gear oil and automatic transmission oil), the composition loses adhesion to magnesium alloy. In addition, since there are available only a few types of acidic silane coupling agents, the composition lacks the freedom of material design and is uneconomical.